1. Field of the Invention
The present invention relates to a power supply unit that includes an auxiliary power supply in addition to a main power supply.
2. Description of the Related Art
In recent years, copiers using an electrophotographic process, printers, facsimile machines, and multifunction products (MFPs) combining a copier, a printer, and a facsimile machine have become multifunctional. Accompanying the increasing multifunctionality, the copiers, the printers, the facsimile machines, and the MFPs have increasingly complex structures and larger maximum power consumption. In addition, amount of power supplied to a fixing heater is increased to reduce wait-time required by an operator and contributing factors from an image forming apparatus itself, such as wait-time required until a fixing device starts and temporary termination of an operation caused by a reduction in fixing temperature during a printing operation or a copying operation.
At the same time, an amount of suppliable power of an ordinary power line is required to be limited. As a result, device design is significantly restricted. To prevent maximum amount of suppliable power of the power line from being exceeded, Japanese Patent Application Laid-open No. 2004-236492 describes a following power supply unit and image forming apparatus. The power supply unit predicts power consumption. When the predicted power consumption exceeds an amount of suppliable power of a main power source, the power supply unit switches power from the main power supply to power from an auxiliary power supply using a switching circuit and supplies a number of loads with the switched power.
Japanese Patent Application Laid-open No. 2005-221674 describes a following image forming apparatus. The image forming apparatus uses a constant-voltage power supply circuit as an auxiliary power supply and sets an output voltage from the auxiliary power supply to be higher than an output voltage from a main power supply. The image forming apparatus applies the output voltage from the auxiliary power supply to a power supply line leading from the main power supply to a load, via a diode that prevents backflow to the main power supply. The image forming apparatus also applies the output voltage from the auxiliary power supply to a power supply line between the diode and the load, via a switch or another diode. The image forming apparatus supplies power from only the auxiliary power supply to the load when the output voltage from the auxiliary power supply is higher than the output voltage from the main power supply.
However, in conventional technology, a power output circuit of a capacitor, namely a power supply circuit supplying power to a load, is a constant-voltage power supply. Therefore, when the power supplied to the load is switched between an output from an AC\DC power supply (main power supply) that is the constant-voltage power supply and an output from an auxiliary power supply that is also the constant-voltage power supply, using a switching circuit, voltage fluctuation occurs during switching because of a difference in output voltages from the two constant-voltage power supplies. When the voltage fluctuation occurs, an operation of a motor to which the power is being supplied becomes unstable. Problems may occur, such as the motor stopping and uneven rotation of the motor. Uneven rotation of the motor causes image abnormality in the image forming apparatus. For example, color shifting occurs in a color image forming apparatus.
Therefore, the present applicant presented a power supply unit in Japanese Patent Application No. 2005-335889, which uses a constant-voltage power supply as a second power supply (auxiliary power supply). The power supply unit connects an output from a first power supply (main power supply) performing constant-voltage control and an output from the second power supply in parallel and simultaneously supplies the output from the first power supply and the output from the second power supply to a load. The power supply unit eliminates switching of the power supply from one power supply to the other and reduces voltage fluctuation caused by the switching.
As described above, the power supply unit simultaneously supplies the output from the first power supply and the output from the second power supply to the load. In the power supply unit, the second power supply supplies an amount of current that is an amount by which a load current exceeds a maximum current indicated to the first power supply. The second power supply is the auxiliary power supply. As a result, the power supply unit requires a means of determining the load current. Therefore, a current detection resistor is added to a power supply line between the load and a parallel connecting point of the output from the first power supply that is the main power supply and the output from the second power supply that is the auxiliary power supply. However, when the current detection resistor is added and load fluctuation occurs, an applied load voltage fluctuates by an amount attributed to the current detection resistor. A resistance value of the current detection resistor can be made small as a method for reducing the fluctuation. However, the fluctuation cannot be reduced further than a fluctuation level of when no current detection resistor is present. A differential amplifier secures a voltage level of a load current detection signal to be inputted to a current indicator at a latter stage. In a load current detector, gain of the differential amplifier becomes too large. Errors in detection currents may increase, and detection accuracy may decrease. When a current sensor (such as a Hall integrated circuit (IC)) that does not use resistive elements is used for load current detection, the fluctuation in the applied load voltage caused by addition of the current sensor is eliminated. However, cost significantly increases.